This invention is directed to a method of determining the exact moment when a material is undergoing a change of state, e.g., in industrial or scientific processes. The invention is more particularly directed to the detection of the exact moment of melting in solids which are being heated to their melting points. It applies also to the detection of "freezing" in liquids which are being cooled to their solidification points.
The invention is especially applicable to the electronics manufacturing industry, particularly in the manufacture of printed circuit boards and the like where molten solder is applied at an electrical connection in order to form a permanent mechanical and electrical bond between two conductors. However, it should be appreciated that there are a wide variety of uses for the invention and it will be described in the context of printed circuit board manufacturing by way of example only.
The process of applying solder to form a permanent mechanical and electrical bond between two conductors on a printed circuit board is carried out in various ways which will be familiar to practitioners of the soldering art and will not be discussed at length here. For small-scale production, solder joints are individually hand-soldered; for large-scale production, an entire circuit board containing hundreds or thousands of solder-joints-to-be can be soldered in one step by wave-soldering or by reflow soldering. In the former, after certain preparatory steps, the board is passed over the surface of a molten solder bath where the solder is caused to adhere to local areas at the intended joints. In reflow soldering, individual solder pads are formed at the desired locations by use of molten solder which is then allowed to solidify. The desired electrical conductors are then placed in mechanical contact with their proper pads and the entire board is raised to the desired temperature either by radiant heating or by various other methods. Careful control is required of heating rates and temperatures.
The preferred embodiment of our invention is addressed to the process of reflow soldering. A particular problem in this process is that all solder joints on a given board do not always have the same amount of solder or of adjoining metal in contact with the solder. The result is that various solder joints will have different heat-input requirements, whereas standard radiant or convective heat-input methods will tend to overheat the smaller joints while underheating the larger ones. This problem is partly overcome by yet another method of reflow soldering, notably vapor phase soldering, but this method is not yet in wide use and it carries other problems which remain to be solved.
In industrial processes such as reflow soldering in which materials are being heat-processed, it is important to know the exact moment when a given solid turns to liquid or vice-versa. It is also advantageous to be able to make this determinaion without making physical contact with the sample and without having to know the radiant emissivity of the surface.